1. FIELD OF THE INVENTION
This invention relates to a synchronization stabilizing circuit and a television signal receiver, a circuit for stabilizing vertical synchronizing signal of a television signal receiver and the television signal receiver.
2. DESCRIPTION OF THE RELATED ART
Heretofore, in transmitting and receiving television signals for example, a synchronizing signal is superimposed on the video signal to be transmitted, in order to completely coordinate the speed and time of scanning line one by one between the transmission side and reception side, which is used for controlling the frequency of horizontal and vertical deflection oscillating circuit of the receiver.
A horizontal synchronizing signal is used to move a scanning point to the left end on a screen again after it has moved from the left end to the right end. On the other hand, a vertical synchronizing signal is used to move a scanning point from to the lowermost end on the screen to the uppermost end after the scanning point on the screen successively has moved from the left end to the right end and scanning for one field has been completed.
The vertical synchronization of an image may be disturbed in weak field reception, or due to the drop-out of the vertical synchronizing signal in specially reproducing a video tape recorder (VTR) and the mixing of noise, etc. To avoid these disturbances, the vertical synchronization follow-up range having some capability for pulling the vertical synchronizing signal is provided and signals in a range other than the follow-up range are ignored as unnecessary signals.
More specifically, as shown in FIG. 1, a follow-up range TR is provided for the vertical synchronizing signal SH. The followed vertical synchronizing pulse Pt is output to the vertical synchronizing signal SH within the follow-up range TR. The vertical synchronizing signal SH which is input to a range other than the follow-up range TR (shown by H0 in the figure) is regarded as an unnecessary signal and ignored. When the vertical synchronizing signal SH is dropped out in the follow-up range TR (shown by H1 in the figure), it is interpolated with the vertical synchronizing pulse P1 to remove the unevenness of jitter or flow of the vertical synchronization on the image.
However, when the follow-up range TR of the vertical synchronizing signal SH is repeated overtime, as shown in FIG. 2, the frequency of the vertical synchronizing signal may considerably deviate from the switching input signal or switching television channel (shown by H2 in the figure). In this case, it takes an extra time to output the vertical synchronizing pulse P2 which is synchronous with the regular vertical synchronizing signal SH. For example, when the follow-up range TR is set to 238.5H-286.5H, a signal having normal frequency takes 217 msec at maximum to pull the regular vertical synchronizing signal SH. In this way, when the frequency of the vertical synchronizing signal SH is considerably deviated, there has been a problem that the regular vertical synchronizing signal SH is misperceived as an unnecessary signal and ignored, so that the vertical synchronization of the image takes extra time to be returned in a regular position and unevenness occurs on the screen.
Further, if the vertical synchronization deviates in a television signal receiver, the blanking may be performed on an image for a predetermined period to remove the unevenness. However, there has been a problem that the disorder of the vertical synchronization appears on a screen when the input signal is switched by an external switch.